Fault-tolerant permanent magnet machine with reconfigurable stator core slot opening and back iron flux paths

ABSTRACT

A permanent magnet (PM) machine includes a plurality of reconfigurable fault condition mechanisms disposed within a stator core portion, the plurality of reconfigurable fault condition mechanisms together automatically reconfigurable to reduce fault currents and internal heat associated with the PM machine during a fault condition. The plurality of reconfigurable fault condition mechanisms are disposed solely within the stator core portion according to one embodiment to automatically reduce stator winding fault currents and internal heat associated with the PM machine during a fault condition. A method of reconfiguring the fault condition mechanisms upon detecting a fault condition includes the steps of 1) selecting the plurality of reconfigurable fault condition mechanisms from a) a plurality of rotatable magnetically anisotropic cylinders disposed both within a stator back iron and stator slot openings, and b) a plurality of rotatable magnetically anisotropic cylinders disposed within a stator back iron and a sliding shield disposed with a stator slot opening portion of the stator core, and 2) reconfiguring the plurality of fault condition mechanisms together to automatically reduce fault currents associated with the PM machine upon detection of a fault condition.

BACKGROUND

The present invention is directed to permanent magnet machines, and moreparticularly to a method of making a permanent magnet machine morefault-tolerant.

Many new aircraft systems are designed to accommodate electrical loadsthat are greater than those on current aircraft systems. The electricalsystem specifications of commercial airliner designs currently beingdeveloped may demand up to twice the electrical power of currentcommercial airliners. This increased electrical power demand must bederived from mechanical power extracted from the engines that power theaircraft. When operating an aircraft engine at relatively low powerlevels, e.g., while idly descending from altitude, extracting thisadditional electrical power from the engine mechanical power may reducethe ability to operate the engine properly.

Traditionally, electrical power is extracted from the high-pressure (HP)engine spool in a gas turbine engine. The relatively high operatingspeed of the HP engine spool makes it an ideal source of mechanicalpower to drive the electrical generators connected to the engine.However, it is desirable to draw power from additional sources withinthe engine, rather than rely solely on the HP engine spool to drive theelectrical generators. The low-pressure (LP) engine spool provides analternate source of power transfer.

PM machines (or generators) are a possible means for extracting electricpower from the LP spool. However, aviation applications require faulttolerance, and as discussed below, PM machines can experience faultsunder certain circumstances and existing techniques for fault tolerantPM generators suffer from drawbacks, such as increased size and weight.

Permanent magnet (PM) machines have high power and torque density. UsingPM machines in applications wherein minimizing the weight is a criticalfactor is therefore advantageous. These applications are wide rangingand include aerospace applications.

One of the key concerns with using PM machines is fault-tolerance sincethe magnets cannot be “turned off” in case of a fault. Traditionally,the use of PM machines has been avoided in applications wherefault-tolerance is a key factor. When PM machines have been used in suchapplications, fault-tolerance has been achieved by paying a penalty inthe form of oversized machines and/or converter designs, or using ahigher number of phases which complicates the control process and addsto the overall system weight and cost.

As is known to those skilled in the art, electrical generators mayutilize permanent magnets (PM) as a primary mechanism to generatemagnetic fields of high magnitudes. Such machines, also termed PMmachines, are formed from other electrical and mechanical components,such as wiring or windings, shafts, bearings and so forth, enabling theconversion of electrical energy from mechanical energy, where in thecase of electrical motors the converse is true. Unlike electromagnetswhich can be controlled, e.g., turned on and off, by electrical energy,PMs always remain on, that is, magnetic fields produced by the PMpersists due to their inherent ferromagnetic properties. Consequently,should an electrical device having a PM experience a fault, it may notbe possible to expediently stop the device because of the persistentmagnetic field of the PM causing the device to keep operating. Suchfaults may be in the form of fault currents produced due to defects inthe stator windings or mechanical faults arising from defective orworn-out mechanical components disposed within the device. Hence, theinability to control the PM during the above mentioned or other relatedfaults may damage the PM machine and/or devices coupled thereto.

Further, fault-tolerant systems currently used in PM machinessubstantially increase the size and weight of these devices limiting thescope of applications in which such PM machines can be employed.Moreover, such fault tolerant systems require cumbersome designs ofcomplicated control systems, substantially increasing the cost of the PMmachine.

In view of the foregoing, it would be advantageous and beneficial toprovide a method for limiting winding currents for all types of faults,especially a turn-to-turn fault associated with a PM machine tosignificantly improve the fault-tolerance capability of the PM machinewithout substantially increasing the size, weight and/or complexity ofthe PM machine.

BRIEF DESCRIPTION OF THE INVENTION

The present invention is directed to a permanent magnet (PM) machinecomprising a plurality of reconfigurable fault condition mechanismsdisposed within a stator core portion, the plurality of reconfigurablefault condition mechanisms together automatically reconfigurable toreduce fault currents and internal heat associated with the PM machineduring a fault condition.

The plurality of reconfigurable fault condition mechanisms are disposedsolely within the stator core portion according to one embodiment toautomatically reduce stator winding fault currents and internal heatassociated with the PM machine during a fault condition.

A method of reconfiguring the fault condition mechanisms upon detectinga fault condition comprises the steps of 1) selecting the plurality ofreconfigurable fault condition mechanisms from a) a plurality ofrotatable magnetically anisotropic cylinders disposed both within astator back iron and stator slot openings, and b) a plurality ofrotatable magnetically anisotropic cylinders disposed within a statorback iron and a sliding shield disposed with a stator slot openingportion of the stator core, and 2) reconfiguring the plurality of faultcondition mechanisms together to automatically reduce fault currentsassociated with the PM machine upon detection of a fault condition.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and features of the present invention and many of theattendant advantages of the present invention will be readilyappreciated as the same become better understood by reference to thefollowing detailed description when considered in connection with theaccompanying drawings in which like reference numerals designate likeparts throughout the figures thereof and wherein:

FIG. 1 illustrates a portion of a permanent magnet (PM) machinedepicting rotatable anisotropic material cylinders in the PM machinestator core slots as well as the stator slot openings under normaloperating conditions according to one embodiment of the presentinvention;

FIG. 2 illustrates a portion of a permanent magnet (PM) machinedepicting rotatable anisotropic material cylinders in the PM machinestator core slots as well as the stator slot openings under a faultcondition according to one embodiment of the present invention;

FIGS. 3 a and 3 b illustrate an actuator or gear assembly for rotatingthe rotatable cylinders shown in FIGS. 1 and 2.

FIG. 4 illustrates portion of a permanent magnet (PM) machine depictingrotatable anisotropic material cylinders in the stator back iron and asliding shield having magnetic and non-magnetic sections in the PMmachine stator slot side during normal operating conditions according toone embodiment of the present invention;

FIG. 5 illustrates portion of a permanent magnet (PM) machine depictingrotatable anisotropic material cylinders in the stator back iron and asliding shield having magnetic and non-magnetic sections in the PMmachine stator slot side during a fault condition according to oneembodiment of the present invention;

FIG. 6 is a block diagram illustrating a general provision forprotection of a permanent magnet generator using active and/or passivedetection of a thermal overload condition and triggering a protectionmechanism actuator according to one embodiment of the present invention;and

FIG. 7 illustrates a conventional permanent magnet machine architecturethat is known in the prior art.

While the above-identified drawing figures set forth alternativeembodiments, other embodiments of the present invention are alsocontemplated, as noted in the discussion. In all cases, this disclosurepresents illustrated embodiments of the present invention by way ofrepresentation and not limitation. Numerous other modifications andembodiments can be devised by those skilled in the art which fall withinthe scope and spirit of the principles of this invention.

DETAILED DESCRIPTION

Conventional PM synchronous electric machines employ permanent magnetsas the magnetic poles of a rotor, around which a stator is disposed. Thestator has a plurality of teeth that face the rotor. Alternatively, themachine may be designed so that the rotor surrounds the stator. Forhigh-speed operation, a retaining sleeve is usually wrapped around themagnets as needed to keep the magnets in place. The retaining sleeve maybe shrink fit upon the magnets to ensure a non-slip fit. Usually theretaining sleeve is made of one whole metallic piece for structuralintegrity. When the coils formed on the stator are energized, a magneticflux is induced by the current through the coils, creatingelectromagnetic forces between the stator and the rotor. Theseelectromagnetic forces contain tangential and/or circumferential forcesthat cause the rotor to rotate.

In order to achieve inherent fault-tolerance in these PM machines, therehas to be complete electromagnetic, thermal, and physical isolationbetween the coils of the various phases. This is achieved by usingfractional-slot concentrated windings where each coil is wound around asingle stator tooth and each stator slot is occupied by one side of acoil. Since slots formed between the teeth and the permanent magnets onthe rotor are spaced from each other, the magnetic flux passing througha tooth will pass through the neighboring tooth in the next moment asthe rotor rotates.

The fault-tolerance techniques discussed herein are not limited to PMmachines with fractional-slot concentrated windings. They can just aseasily be applied to any PM machine with any winding configuration toachieve the desired results.

A conventional PM machine that is known in the art is shown in FIG. 7 toprovide a background regarding PM machine architecture before describingseveral embodiments for implementing a synchronous permanent magnetmachine that is fault-tolerant, and with particular focus onturn-to-turn faults, with reference to FIGS. 1-6 herein below.

As can be seen in FIG. 7, a PM machine 1 contains a plurality of magnets2 provided in a radial arrangement upon a back iron 3 that is disposedaround a shaft (not shown). The back iron 3 is also known as a yoke. Themagnets 2 are surrounded by a retaining sleeve 4. A stator 5 surroundsthe retaining sleeve 4 and is separated from the magnets 2 by a gap 6.The stator 5 has a plurality of radially disposed teeth 7 that formstator slots 8. The teeth 7 are wound with coils 9 that substantiallyfill the stator slots 8.

Looking now at FIGS. 1 and 2, there is shown, a portion of a permanentmagnet machine depicting rotatable cylinders 10. The rotatable cylinders10 are constructed of a magnetically anisotropic material. Eachmagnetically anisotropic cylinder can be implemented by forming thecylinder from, for example, a magnetically anisotropic material or froma plurality of magnetic laminations. These laminations can be, forexample, any grade of silicon-steel laminations (e.g., M19, M23, . . . ,etc.) or any grade of iron-cobalt laminations. The magneticallyanisotropic rotatable cylinders 10 are located both in permanent magnetmachine stator core slot openings 12 of the stator core 14 as well as astator back iron 11 according to one embodiment of the presentinvention. The orientation of the magnetically anisotropic material ormagnetic laminations then either impedes or allows a flux path throughthe slot openings 12 or through the stator back iron (yoke) 11. Therotatable magnetically anisotropic (laminated magnetic) cylinders 10 canbe seen in FIG. 1 to be oriented in a direction to conduct a normalmagnetic flux path 16 through the stator core back iron (yoke) 11 undernormal operating conditions. Under fault conditions, all rotatablemagnetically anisotropic cylinders 10 are rotated to simultaneouslyimpede or interrupt the normal magnetic flux path 16 in the stator backiron 11 and allow a flux path through the slot openings 12.

FIG. 2 depicts the new flux path 18 under a fault condition and showsthe new flux path 18 does not pass through the back iron 11 of thepermanent magnet machine. The rotatable anisotropic cylinders 10 in thestator back iron 11 are disengaged to block the normal flux path(orthogonal to the flux path) 16. In this manner, the rotatableanisotropic cylinders 10 in the stator core slots 12 are rotated 90°under fault conditions to allow a flux path through the slot openingsand thus reduce the magnetic flux coupling the stator windings and limitthe fault current.

FIGS. 3 a and 3 b illustrate actuation of the rotatable anisotropiccylinders 10 depicted in FIGS. 1 and 2. Rotation of the rotatableanisotropic cylinders 10 is implemented via an actuator or gear assembly20. The actuator or gear assembly 20 is affixed on permanent magnetmachine end plates (not shown) in one embodiment. Many types ofactuators and gear assemblies suitable for implementing this structureare easily constructed by those skilled in mechanical engineering; andso actuators and gear assemblies are not discussed in any detail hereinto preserve brevity and provide clarity in describing the particularembodiments herein. Under normal operation, the rotatable magneticallyanisotropic cylinders 10 in the stator back iron 11 are engaged, whilethe rotatable magnetically anisotropic cylinders 10 in the stator slotopenings 12 are disengaged to provide a normal flux path 16 through theback iron 11 such as depicted in FIG. 1.

During a fault condition, the rotatable magnetically anisotropiccylinders 10 in the stator back iron 11 are disengaged; while therotatable magnetically anisotropic cylinders 10 in the stator slotopenings 12 are engaged by the actuator or gear assembly 20 as seen inFIGS. 3 a and 3 b, to rotate the rotatable anisotropic cylinders 10 byapproximately 90° to impede or block the normal flux path 16, therebyshunting the magnetic flux away from the windings via a new flux path 18as shown in FIG. 2, and reducing the fault currents.

FIGS. 4 and 5 illustrate a sliding shield 45 in the stator slot openingside of a permanent magnet (PM) machine stator core 14. Sliding shield45 has magnetic sections 52 and nonmagnetic sections 54. In oneembodiment, a plurality of axial-laminated portions are inserted, withsolid pieces of nonmagnetic material inserted between each laminatedportion. The laminated portions can be constructed, for example, usingthe same, but not limited to, materials used for the rotatablemagnetically anisotropic cylinders. The sliding shield 45, according toone embodiment, can be made of a dual-phase magnetic material where thenonmagnetic sections are heat treated. The magnetic sections can also beconstructed, for example, of a magnetically anisotropic material or canoptionally be constructed of magnetic laminations. During normaloperation as shown in FIG. 4, the sliding shield 45 is in itsconventional operating mode in which the nonmagnetic sections 54 arealigned to impede a flux path through the stator core slot openings 12and thus allow flux to pass through the normal flux path 16 through thestator back iron 11.

With continued reference to FIGS. 4 and 5, stator core 14 can be seen toalso have a plurality of rotatable cylinders such as discussed hereinbefore with reference to FIGS. 1 and 2, disposed within the back iron11. As shown in FIG. 4, the sliding shield 45 is positioned such thatthe nonmagnetic sections 54 are aligned with the slot openings 12 duringnormal fault-free operation to impede a flux path through the slotopenings 12; while the rotatable cylinders 10 are rotated to conduct aflux path through the back iron 11 during fault-free operation.

FIG. 5 illustrates the sliding shield 45 and the rotatable cylinders 10during a fault condition in which the sliding shield 45 is positionedsuch that the magnetic material sections 52 are aligned with and providea flux path through the slot openings 12, while the rotatable cylinders10 in the back iron 11 are rotated to impede the flux path through theback iron 11. The magnetically anisotropic material may optionally bereplaced with laminated magnetic portions, as stated herein before.

If a localized electrical fault occurs in the stator core 14 of thepermanent magnet machine, excitation provided by the permanent magnetrotor 21 can cause significant overload current to flow, as describedherein before. Localized heating will occur in this case. When theforegoing localized heating occurs, the heat generated at the internalstator core 14 fault will be detected via an active or passive thermaloverload detector mechanism such as described further herein below withreference to FIG. 6. The thermal overload detector mechanism will thenactivate movement of the sliding shield 45 such that the magneticsections 52 now create a shunt across the stator core slot openings 12to divert more flux through flux path 18 through the stator core slotopenings 12, and less flux through the normal flux path through thestator back iron 11 thus reduce the magnetic flux coupling with thestator windings and limit the fault current. In similar fashion, thethermal overload detector mechanism will activate rotation of thecylinders 10 in the back iron 11 to provide a flux path during normalfault-free operation. The thermal overload detector mechanism will thenreorient the rotatable cylinders 10 during a fault condition to impede aflux path through the back iron 11.

FIG. 6 is a block diagram illustrating a permanent magnet machine (i.e.generator) 50 using active and/or passive detection of a thermaloverload condition, and triggering a protection mechanism actuator 20according to one embodiment of the present invention. The permanentmagnet machine 50 is controlled in response to commands from a generatorcontroller 53 that senses one or more loads 55 supplied by the machine50. The generator controller 53 is also in communication with an activethermal overload detection system 56 that operates to sense operatingpoint conditions that are conducive to machine 50 overloading. Manytypes of active thermal overload detection methods and systems suitablefor implementing the requisite active thermal overload detection system56 are known in the art, and so further details of thermal overloaddetection systems will not be discussed herein.

When the active thermal overload detection system 56 detects anoperating condition that exceeds one or more desired or predeterminedoperating condition set points, the active thermal overload detectionsystem 56 sends one or more command signals to the protective mechanismactuator 20. The protective mechanism actuator 20 then operates inresponse to the command signal(s) to operate the rotatable cylinders 10and the sliding shield 45 shown in FIGS. 1-2 and 4-5 respectively asdescribed herein before.

With continued reference now to FIG. 6, a passive thermal overloaddetection system (sensor) 60 is configured to directly sense thermalconditions of the permanent magnet machine (generator) 50. When thepassive thermal overload detection system 60 is subjected to anoperating condition that exceeds one or more desired or predeterminedoperating condition set points, the passive thermal overload detectionsystem 60 physical state is altered. This changed physical state isdetected by the protective mechanism actuator 20. The protectivemechanism actuator 20 then operates in response to the altered physicalstate to operate the rotatable cylinders 10 and the sliding shield 45shown in FIGS. 1-2 and 4-5 respectively as described herein before.

In summary explanation, methods for improving the fault-tolerance of PMmachines have been described to include various electrical, mechanical,hydraulic or thermal solutions that provide flexibility in choosing theoptimal PM machine architecture from a system point of view. Thesesolutions include, but are not limited to 1) rotatable anisotropic orlaminated magnetic cylinders 10 in the stator core slot openings 12 tointerrupt the stator flux through the stator back iron 11 under faultconditions, 2) a sliding shield in the stator core slot opening sidethat operates to impede a flux path through the stator back iron 11under fault conditions, and 3) combining desired features describedabove as necessary to achieve desired system performance, reliability,cost, size, specifications/requirements, and so on.

A key feature of the embodiments described herein before include theprovision of a fault tolerant permanent magnet machine that is morerobust than permanent magnet machines known in the art that employ moreconventional types of fault sensing mechanisms, actuators, controllers,and so on.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A permanent magnet (PM) machine comprising: a stator core portion; arotor core portion; and a plurality of reconfigurable fault conditionmechanisms disposed solely within the stator core portion, the pluralityof reconfigurable fault condition mechanisms together automaticallyreconfigurable to reduce fault currents and internal heat associatedwith the PM machine during a fault condition, wherein the plurality offault condition mechanisms comprise a plurality of rotatable cylinderscomprising anisotropic material or magnetic laminations disposed bothwithin stator slot openings and a stator back iron of the stator coreportion.
 2. The PM machine according to claim 1 further comprising anactuator or gear assembly configured to rotate the plurality ofrotatable cylinders during a PM machine fault condition.
 3. The PMmachine according to claim 2, wherein the actuator or gear assembly isresponsive to electrical signals generated via an active thermaloverload detector.
 4. The PM machine according to claim 2, wherein theactuator or gear assembly is responsive to electrical signals generatedvia a passive thermal overload detector.
 5. The PM machine according toclaim 1, wherein the rotatable cylinders are oriented during normal PMmachine operation to conduct a flux path though the stator back iron andimpede a flux path though the stator slot openings.
 6. The PM machineaccording to claim 1, wherein the rotatable cylinders are orientedduring a PM machine fault condition to impede a flux path though thestator back iron and conduct a flux path though the stator slotopenings.
 7. A permanent magnet (PM) machine comprising: a stator coreportion; a rotor core portion; and a plurality of reconfigurable faultcondition mechanisms disposed solely within the stator core portion, theplurality of reconfigurable fault condition mechanisms togetherautomatically reconfigurable to reduce fault currents and internal heatassociated with the PM machine during a fault condition, wherein theplurality of fault condition mechanisms comprise a plurality ofrotatable cylinders comprising anisotropic material or magneticlaminations disposed within a stator back iron and a sliding shielddisposed within a slot opening portion of the stator core portion. 8.The PM machine according to claim 7 further comprising an actuator orgear assembly configured to rotate the plurality of rotatable cylindersand move the sliding shield during a PM machine fault condition.
 9. ThePM machine according to claim 8, wherein the actuator or gear assemblyis responsive to electrical signals generated via an active thermaloverload detector.
 10. The PM machine according to claim 8, wherein theactuator or gear assembly is responsive to electrical signals generatedvia a passive thermal overload detector.
 11. The PM machine according toclaim 7, wherein the rotatable cylinders are oriented during normalfault-free PM machine operation to conduct a flux path though the statorback iron, and further wherein the sliding shield is moved during normalfault-free PM machine operation to impede a flux path though stator slotopenings.
 12. The PM machine according to claim 7, wherein the rotatablecylinders are oriented during a PM machine fault condition to impede aflux path though the stator back iron, and further wherein the slidingshield is moved during a PM machine fault condition to conduct a fluxpath though stator slot openings.
 13. A permanent magnet (PM) machinecomprising a stator portion having a plurality of fault conditionmechanisms disposed therein, the plurality of fault condition mechanismsautomatically reconfigurable in combination to reduce stator windingfault currents and internal heat associated with the PM machine during afault condition, wherein the plurality of fault condition mechanismscomprises at least one rotatable cylinder disposed within a back iron ofthe stator portion, and further comprises a sliding shield disposedwithin a stator slot opening portion of the stator portion.
 14. The PMmachine according to claim 13, wherein the sliding shield comprises aplurality of sections selected from a dual-phase magnetic material,magnetic laminations, and magnetically anisotropic material.
 15. The PMmachine according to claim 13, wherein the at least one rotatablecylinder is constructed of magnetically anisotropic material.
 16. The PMmachine according to claim 13, wherein the at least one rotatablecylinder is oriented during normal fault-free PM machine operation toconduct a flux path though the stator back iron, and further wherein thesliding shield is positioned during normal fault-free PM machineoperation to impede a flux path though stator slot openings.
 17. The PMmachine according to claim 13 further comprising an actuator or gearassembly configured to rotate the at least one rotatable cylinder andmove the sliding shield during a PM machine fault condition.
 18. The PMmachine according to claim 17, wherein the actuator or gear assembly isresponsive to electrical signals generated via an active thermaloverload detector.
 19. The PM machine according to claim 17, wherein theactuator or gear assembly is responsive to changed physical conditionassociated with a passive thermal overload detector.
 20. A method ofreconfiguring a permanent magnet (PM) machine upon detecting a faultcondition, the method comprising the steps of: providing permanentmagnet (PM) machine with a stator core comprising a plurality of faultcondition mechanisms disposed therein, the plurality of mechanismsselected from a plurality of rotatable magnetically anisotropiccylinders disposed both within a stator back iron and stator slotopenings, and a plurality of rotatable magnetically anisotropiccylinders disposed within a stator back iron and a sliding shielddisposed with a stator slot opening portion of the stator core; andreconfiguring the plurality of fault condition mechanisms together toautomatically reduce fault currents associated with the PM machine upondetection of a fault condition.
 21. The method according to claim 20,wherein the step of reconfiguring the plurality of fault conditionmechanisms comprises rotating the plurality of magnetically anisotropiccylinders, such that the plurality of magnetically anisotropic cylindersimpede a normal PM machine flux path though a back iron of the statorcore, and moving the sliding shield to conduct a flux path though thestator core slots.
 22. The method according to claim 20, wherein thestep of reconfiguring the plurality of fault condition mechanismscomprises rotating the plurality of magnetically anisotropic cylinderssuch that the magnetically anisotropic cylinders in the back iron impedea flux path though a back iron of the stator core, and further such thatthe magnetically anisotropic cylinders in the stator slots conduct aflux path though the stator core slots.